Problem: Simplify the following expression: $z = \dfrac{9}{7a} - \dfrac{7}{2a}$
Answer: In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $7a$ and $2a$ $\lcm(7a, 2a) = 14a$ $ z = \dfrac{2}{2} \cdot \dfrac{9}{7a} - \dfrac{7}{7} \cdot \dfrac{7}{2a} $ $z = \dfrac{18}{14a} - \dfrac{49}{14a}$ $z = \dfrac{18 -49}{14a}$ $z = \dfrac{-31}{14a}$